1. Field of the Invention
The present invention relates to a cam follower that engages with a cam (groove) for advancing and retracting a frame member of a lens barrel and the construction of the lens barrel to which the cam follower is applied.
2. Description of the Related Art
As a conventional lens barrel, a lens barrel in which a frame member is advancingly and retractingly driven in the optical axis direction by a cam groove and a cam follower has been known. On the lens barrel disclosed, for example, in Japanese Patent Application Laid-Open Publication No. 2000-131588, the cam groove provided in a cam frame and the cam follower that is fixed to a moving frame member and slidably fitted in the cam groove are provided, and the moving frame member is advancingly and retractingly driven along with the rotation of the cam frame. The cam follower is in slidable contact with the cam groove, and has a slidably contacting portion formed by a taper surface and a solid shaft portion fittingly fixed by press fitting or bonding in a shaft hole in the moving frame member.
For example, when being incorporated in a camera as a zoom lens, in the photographing state, the conventional lens barrel becomes in the state in which a front group lens frame members of the lens barrel are put out in succession from a camera external body to the front. Assuming that the cam follower is made of a metal and the moving frame member is made of a synthetic resin, in case of accident in which the camera is dropped and the front group lens frame member is brought into contact with an obstacle, naturally, an impact force (external force) along the optical axis O direction acts on the cam groove and the cam follower.
FIG. 8 shows a cross section around the cam groove and the cam follower in the conventional lens barrel. In this figure, a cam follower 53 on a moving frame 52 side is slidably fitted in a cam groove 51a in a cam frame 51. The state of stress concentration in a shaft hole of the moving frame at the time when a predetermined impact force (external force) F0 acts on the lens barrel is analyzed by using a computer, and a stress distribution formed around the shaft hole is additionally shown in FIG. 8.
Specifically, the cam follower 53 includes a slidably contacting portion 53a formed by a taper surface, which comes into slidable contact with the cam groove 51a, and a solid shaft portion 53b. The shaft portion 53b is fittingly fixed in a shaft hole 52a of the moving frame 52. According to the aforementioned computer analysis, in the case where the impact force F0 acts, as shown in FIG. 8, stresses a1 to a7 are created on the side of the shaft portion 53b on which the external force acts, and stresses a3 to a8 are created on the opposite side thereof. The stress aN in each zone shows a stress value that decreases gradually in the order of N=1 to N=8.